Cargo and weapons bound for a naval vessel or other type of ship are normally packaged for transportation and stowage in one of two ways: goods are either secured to a pallet or are enclosed in a shipping container. Based on a typical inventory of weapons and stores aboard a current-generation aircraft carrier or other surface combatant, most pallets measure 44 inches in length by 40 inches in height and can weigh as much as 3,800 pounds. Containerized loads, in which the cargo or weapons are fully enclosed in a rigid box, can weigh up to 9,640 pounds, with lengths up to 312 inches. Individual pallets and containers of all types and sizes are handled many times by various crews and equipment and may be restowed in the holds of several different ships before reaching their ultimate point of use.
Such palletized and containerized cargo and weapons payloads are generally first moved from locations in pierside warehouses or weapons storage depots to staging areas on a dock using forklift trucks. They are then hoisted onto the top deck of a shuttle ship or a specialized cargo vessel called an Underway Replenishment (UNREP) ship using conventional cranes. Once aboard the UNREP ship, the pallets and containers are again moved with forklifts, pallet movers, or sometimes cranes to one of several elevators, where they are lowered for stowage into a hold or magazine on one of the vessel's multiple cargo decks.
After descending to the appropriate hold or magazine, each pallet or container is removed from an elevator platform using another forklift truck and is deposited at its particular stowage site in the storeroom, where it is usually stacked on identical pallets or containers to the maximum height permitted by either container capacity or the height of the storeroom ceiling. Each individual load or stack is then manually secured to the deck for safe transit at sea using tie-down straps, chains, nets or blocking. When the time comes to transfer the pallets and containers from the UNREP ship to a surface combatant during transit at sea, the procedure is reversed. After the cargo is delivered to the combatant ship via connected replenishment gear or aircraft, the same procedures are again employed, using a series of lift trucks and elevators to restow the pallets and containers in holds and magazines located below decks.
This stowage and retrieval process is extremely time-consuming, manpower-intensive, and inefficient. For example, during the cargo retrieval process, forklift operators in each hold or weapons magazine must select the pallet or container that has been ordered, manually remove the tie-down straps, chains, nets or other restraining devices that were previously installed to secure it to the hold deck for safe transit at sea, and then pick up the load, maneuver it between the other stored cargo, and deliver it to the elevator trunk. When the elevator platform becomes available, the forklift drives onto the platform and deposits the payload. The elevator often must wait until several of the weapons or cargo payloads requested from that magazine or hold have been acquired and loaded before it can deliver the goods to their destination, delaying parallel activities in the other magazines and holds that the elevator services.
Forklift trucks, which are typically the prime movers for horizontal operations in this entire sequence of events, have certain intrinsic disadvantages for this application. First, they require aisles to be cleared within which to maneuver the payloads, and space to access each with their tines, so the cargo in each hold or magazine is repeatedly rearranged to acquire requested payloads. A considerable amount of floor space must be left vacant to provide sufficient maneuvering room for the forklifts and for temporary cargo staging areas. As a result, payloads cannot be stowed as densely as desired. Second, forklift trucks are by-nature quite heavy themselves and thus place undue stress on the elevator platform and its actuator system when driven onto the freight elevator carrying individual payloads. Third, as discussed, payloads must be unloaded from or loaded onto the freight elevator platform one at a time, so the elevator must wait until each is individually stowed or retrieved. Fourth, forklifts have proved to be quite maintenance-intensive and costly over their service life. Finally, this cargo and weapons stowage and retrieval process must often be performed in high seas, where even the largest surface vessels, such as aircraft carriers, pitch and roll violently. In certain sea states, handling large and heavy palletized and containerized loads with forklift trucks becomes unsafe and the process must be stopped.
Conventional “rack-and-aisle” automated storage and retrieval systems used today in land-based warehouses also have significant limitations. First, these systems are capable of handling payloads of only one size and shape, typically pallets. Second, in order to achieve selective access, i.e., the ability to access any individual payload contained in the system, one fixed, empty aisle must be provided between every two storage racks to provide access to every cargo unit, or empty rack space must be reserved to allow payloads to be shuffled from one rack to another. In either case, high storage density cannot be achieved. Finally, these industrial warehousing systems are not designed for shipboard applications in which the cargo contained is subject to high dynamic loads caused by ship motion and must be restrained at all times.
Despite continuing efforts on the part of the Navy and commercial operators to maximize efficiency in transporting, handling and stowing palletized and containerized cargo and weapons of various sizes and shapes at sea, current systems have limitations in stowage density, speed of access, and securing of payloads. Accordingly, automated stowage and retrieval systems are desired that achieve high three-dimensional stowage density within a given hold or magazine, that permit any payload contained in the storeroom to be accessed, loaded and unloaded on associated service elevators quickly, and/or that automatically secure those payloads for transit in rough seas.